Salmonellae cause an estimated 150 million cases of gastroenteritis and 25 million cases of invasive disease (enteric fever and non-typhoidal bacteremia), leading to 300,000 deaths per year. Salmonella manipulates multiple host cellular pathways through secreted effector proteins, but the molecular functions of most effectors, especially in animals and humans, remain poorly understood. A thorough characterization of how Salmonella effectors function and the pathways they target is required to understand how effectors impact infection and their long-term consequences on chronic inflammatory conditions, such as inflammatory bowel disease (IBD). IBD is an immune-mediated disease and whether IBD is affected by previous Salmonella infection is unclear, with conflicting evidence from both epidemiological and mouse studies. To understand how acute Salmonella infections may impact long-term immune modulation, studies are needed to determine how specific Salmonella effectors target host immune regulatory pathways, whether these changes persist beyond infection, and if these changes modulate risk or severity of disease. Through comparative genomics of Salmonella serovars, we recently identified a novel prophage-encoded Type III secreted effector: Salmonella anti-inflammatory response activator (SarA). SarA is the only Salmonella effector demonstrated to be necessary and sufficient to activate STAT3 (signal transducer and activator of transcription-3), a key transcription factor that regulates immune cell proliferation, development, and autoimmune conditions including IBD. SarA-mediated manipulation of STAT3 reprograms transcription in host cells and increases virulence in mice. We hypothesize SarA has both direct effects on cells injected with the effector and secondary consequences due to STAT3 target genes (such as the anti-inflammatory cytokine IL-10) that may cause persistent changes after infection has cleared. Therefore, the objective of this application is to determine how SarA activates STAT3 and affects immune cell populations during and after infection. Based on preliminary data, we hypothesize that SarA directly binds STAT3 and cofactors to assemble a STAT3- activating platform. Activation of this pathway phosphorylates STAT3 in multiple cell types, but it is unknown which cells are targeted in vivo and what the consequences of this activation are during and after infection. Therefore, we propose Specific Aims to 1) determine how SarA drives STAT3 activation through biochemical and genetic approaches and 2) determine the effects of SarA on immune cell populations and long-term consequences for the host, including severity of intestinal inflammation in colitis. Following completion of these aims, we will have determined how SarA mediates STAT3 signaling and how/if these signaling events alter immune function during and after infection. Revealing these mechanisms could lead to new therapeutic strategies for treating salmonellosis, as well as other STAT3-dependent pathological conditions including autoimmune diseases, cancer, and other infectious diseases.